docs: misc cleanups

Here's a few RST cleanups for the docs.

The main thing this does, is get rid of what appears to be accidental uses of the "default role" in RST (e.g using foo instead of foo). This causes the <cite>-tag to be emitted in the rendered HTML, which I strongly doubt is something we ever really want.

The motivation for doing this, is that I would like to switch the default role to the :c:expr:-role instead in the config. This gives us a nice short-hand for writing C expressions, which is something that's super-useful in the Mesa docs, which often refers to C code. But that's a second step. Let's clean this up first, to pave the way for future changes.